Conventionally, an aircraft is provided with wings that provide it with lift, the wings being disposed on either side of the fuselage of an airplane, for example.
When the aircraft is moving, the top surface of the wing, also known as the “suction side” surface, is subjected to suction forces. In contrast, the bottom face of the wing, also known as the “pressure side” surface, is subjected to pressure forces.
The aerodynamic resultant of these pressure and suction forces then gives rise firstly to lift perpendicular to the relative wind, thereby lifting the wing, and secondly to drag that slows down the profile and pointlessly absorbs energy, with a portion of this drag being known as “compressibility drag” when there are supersonic zones present that engage the wing.
It should be observed that compressibility drag is maintained at a relatively constant level until the wing reaches a determined speed relative to the air flow, corresponding to the drag divergence Mach number of the wing. Mach number is equal to the quotient of speed divided by the speed of sound, and the drag divergence Mach number is defined by the person skilled in the art as the Mach number from which compressibility drag diverges and increases suddenly.
Under such conditions, the design of an aircraft wing is the result of a compromise between various factors such as the desired lift, the drag generated by the wing, or indeed the structural strength of the wing.
Nevertheless, over time, the definition of an aircraft is called on to change in order to satisfy user requirements, e.g. in order to increase its payload. Such changes may require an increase in the lift generated by the wing.
In general, the person skilled in the art solves such a problem by increasing the overall angle of attack of the aircraft.
Although initially effective, that technique sometimes leads to unexpected drawbacks. The increase in the lift of the wing is accompanied by a corresponding increase in its drag, and in particular its compressibility drag.
Unfortunately, compressibility drag diverges spectacularly starting from a defined speed for the flow of air over the suction surface of the wing, corresponding to the drag divergence Mach number. It is then possible, in certain critical zones of the wing, for the increase in lift not to compensate the harmful effects created by the large increase in compressibility drag.
One solution to that problem would be to cause the aircraft to fly at a lower altitude. However that solution leads to a significant degradation in aircraft flight conditions.
Thus, in order to avoid degrading such flight conditions, the person skilled in the art has until now had only one solution for countering this phenomenon. That solution consists in changing the shape of the wing so as to obtain once more an acceptable compromise between lift and compressibility drag.
That situation is nevertheless penalizing, since designing a new wing and making it can be very expensive. In addition, the same goes to modifying existing aircraft that are to be fitted with a new wing.
A method is known for reducing compressibility drag in at least one critical zone of an aircraft wing, which method is remarkable wherein at least one container is placed under the wing, the container having a rear portion in the vicinity of its trailing edge that tapers in a direction going away from the leading edge of the container towards its trailing edge. That rear portion of the container is arranged at least in part upstream from the leading edge of the wing so that the container slows down the speed of the air upstream from the critical zone that is to be treated so as to reduce the compressibility drag of the wing.
For example, document DE 932410 discloses a method of reducing the compressibility drag of at least one critical zone of an aircraft wing, under which at least one container is fastened that has a rear portion in the vicinity of its trailing edge that tapers in a direction going away from the leading edge of the container towards its trailing edge.
The rear portion is arranged at least in part upstream from the leading edge of the wing so that the container slows down the speed of air upstream from the critical zone in order to reduce its compressibility drag.
In addition, said container is fastened under the wing via fastener means having a front end that projects as much as possible from the leading edge of the wing through a distance L1 of the order of the length L of the container.